This invention relates to solids-liquid separating centrifuges of the continuous type in which a bowl and a helically bladed conveyor are rotated about a common axis in the same direction but at a differential speed to discharge a solids fraction continuously through a reduced diameter outlet, usually a conical end of the bowl. More particularly, the invention concerns the provision of such centrifuges with means enabling the effective separation thereby of the solids of a liquid slurry thereof into two or more separate output fractions of generally different characteristics.
Slurries treated in such centrifuges frequently contain solid particles which vary over a large range in size and other characteristics. In many cases, a fraction of these particles has characteristics which are undesired in the solids recovery sought, such as the generally finer and lighter particle fraction in some slurries, known as "slimes". Slimes not only foul the desired separated solids output, but also interfere seriously with the separation operation itself, since slimes are difficult to convey properly even if kept suspended in slurry liquid up to the reduced diameter point of discharge. Such suspension generally results in a solids cake output wetter than desired, and yet, if it is not maintained, serious consequences may follow, such as choking of the centrifuge and/or fouling of the effluent fraction. Throughput of the machine is curtailed well below capacity.
Operation difficulties with centrifuges of the type concerned, occasioned by slimes in the slurry, have led to the alternative use of centrifuges of the so-called "nozzle" type, wherein all the solids are discharged through nozzles in a solid bowl. But nozzle type centrifuges have other problems such as wet solids cake discharge and plugging of the nozzles by the larger size particles which have to pass through them.
As a compromise, it has been proposed in the prior art, e.g., U.S. Pat. No. 2,766,930, to modify a centrifuge of the conical end discharge type by including nozzles in an area of the bowl adjacent the cylindrical end opposite the discharge end, where settling of the slimes relative to the coarser fraction is greatest, and to reverse the pitch of a portion of the conveyor blade between the nozzles and the conical discharge end, so that this conveyor part conveys away from the discharge end. Thus, all the solids in the area of the bowl containing the nozzles and the oppositely pitched conveyor portions at opposite sides thereof convey to the nozzles, and all of the solids settling in this part of the bowl have to exit through the nozzles, as they do in a conventional nozzle type centrifuge. So far as known, this proposal has not had significant commercial acceptance, probably because, while it may rid the slurry of part of the slimes fraction that would otherwise be discharged from the conical end, it also involves considerable loss of desired solids settling in the nozzle-equipped area, as well as plugging of the nozzles by the larger particles, since all settled particles in the area served by the nozzles must exit through them.
A similar prior art suggestion utilizes what is in effect a conventional nozzle type cylindrical centrifuge as one end of the bowl, and a conical end discharge centrifuge as the other end of the bowl, as in German Pat. No. 917,777, having a similar mode of operation and practical deficiencies. Still another prior art suggestion has been the provision of nozzles at the end of the bowl, opposite the conical end, from which the effluent discharges, having upstanding inlet pipes beyond the conveyor close to the level of discharge of the liquid fraction over the effluent weir (see German Pat. No. 924,020). Such an arrangement is only effective to remove solids which have not settled and would otherwise go out with the effluent.